1. Field of the Invention
This invention relates to a remote sensor apparatus and systems and, more particularly, to an improved, electrically isolated remote sensor apparatus which is optically coupled for both electrical power supply and signal transmission, and to a system incorporating same.
2. State of the Relevant Art
It is known to use fiber optic signal conductors, i.e., a fiber optic cable, to achieve electrical isolation of sensors which are required to operate in electrically noisy environments and/or at high voltages, and thereby to avoid the electrical interference which otherwise would be produced if conventional copper wiring were employed as the signal conductor. The most common application involves the use of an optical fiber cable to transmit sensor data from a remote sensor to a central station or control room. Most sensors, however, require electrical power from an external source, which often is not available at the site of the remote sensor. Moreover, to provide the sensor with power through copper or other conventional electrical conductors would defeat the isolation afforded by the fiber optic cable.
A number of solutions to the power problem have been proposed but none so far has proven to be entirely successful. In one, a long life battery is coupled to the sensor. In another, a solar cell is powered by light transmitted through a fiber optic cable.
In yet another, a common light-emitting diode (LED) is used to provide both power and sensor data return; this is desirable, because a common fiber optic cable optically coupled to the LED both powers the sensor and receives the sensor data for return transmission. The LED is operated in a bilateral mode: light is delivered to the LED which, by means of the photovoltaic effect, produces electrical power which is supplied to the sensor unit, which then drives the LED in its normal light-emitting mode. In a specific such embodiment, the LED is driven by a constant light source coupled to it through the fiber optic cable, and the resulting current is used to charge a capacitor. The capacitor energy then is used to power the sensor for a small period of time and to power the LED for emitting light pulses representing the value of the sensor parameter. Even though only extremely small amounts of power can be transmitted in this way, a sufficient quantity of power for the sensor and LED can be achieved by only powering the sensor for a small amount of time and charging the capacitor for most of the time. Implementation has been difficult and complex due to the fact that it requires circuits to switch from the LED to the sensor, control circuits, and data encoding circuits, all of which must be implemented in the remote sensor.
Despite advances made in fiber optic technology, currently available sensors and monitoring systems have failed to satisfy critical needs in the industry. The need for an effective electrical power source for an optically coupled remote sensor is particularly acute in the field of steam turbine power generators, which require a large number of sensors placed at remote sites for accurately detecting critical system operating parameters. There thus exists a continuing need for reliable and effective such sensors and monitoring systems.